U.S. patent number 5,072,786 [Application Number 07/558,591] was granted by the patent office on 1991-12-17 for anti-vibration support of u-bend flow tubes in a nuclear steam generator.
This patent grant is currently assigned to Electric Power Research Institute, Inc.. Invention is credited to Willliam J. Wachter.
United States Patent |
5,072,786 |
Wachter |
December 17, 1991 |
Anti-vibration support of U-bend flow tubes in a nuclear steam
generator
Abstract
An apparatus and a method for eliminating the gap between a flow
tube and a respective support in a U-bend of a nuclear steam
generator whereby the flow tube is disposed in the correct position
and whereby fretting, corrosion and vibration are substantially
eliminated. Support bars are provided between the columns of tubes,
and each support bar carries a plurality of springs. Each spring
resiliently engages a respective tube thereby eliminating the gap
between the tube and the respective support. Each spring further
has an elastic constant which is larger than the bending elastic
constant of the tube, and the spring flexibly accommodates
deformation during insertion of the support bar thereby assuring
ease of installation.
Inventors: |
Wachter; Willliam J. (Wexford,
PA) |
Assignee: |
Electric Power Research Institute,
Inc. (Palo Alto, CA)
|
Family
ID: |
24230146 |
Appl.
No.: |
07/558,591 |
Filed: |
July 27, 1990 |
Current U.S.
Class: |
165/69;
165/162 |
Current CPC
Class: |
F22B
37/206 (20130101); F28F 9/0132 (20130101) |
Current International
Class: |
F22B
37/20 (20060101); F28F 9/007 (20060101); F28F
9/013 (20060101); F22B 37/00 (20060101); F28F
007/00 (); F28D 007/16 () |
Field of
Search: |
;165/69,162,172 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Rivell; John
Assistant Examiner: Leo; L. R.
Attorney, Agent or Firm: Bloom; Leonard
Claims
What is claimed is:
1. In a heat exchanger having a plurality of spaced-apart tubes and
further having at least one support bar disposed intermediately of
the respective tubes, the improvement in reducing vibration of the
tubes against the support bar, comprising a plurality of hairpin
springs carried by the support bar, the hairpin springs being
substantially parallel to each other and spaced-apart from each
other, each of the hairpin springs including a bight portion and
further including a pair of legs, each of the legs having an end
portion secured to the support bar, and further having a bowed
portion intermediately of the end portion and the bight portion,
respectively, and each bowed portion resiliently engaging a
respective tube, thereby substantially eliminating the vibration of
the tubes in the heat exchanger.
2. The improvement of claim 1, further including a central
longitudinal stem to which the respective bight portions of the
plurality of hairpin springs are joined, and the support bar having
a longitudinal edge disposed adjacent to the central longitudinal
stem.
3. The improvement of claim 1, further comprising a plurality of
support bars, such that a respective support bar is disposed
between a pair of tubes, including a first tube and a second tube,
each of the tubes having a spring constant defined as H.sub.t, a
first spring carried by the support bar and resilient engaging the
first tube, a second spring carried by the support bar resiliently
engaging the second tube, each spring having a substantially equal
spring constant defined as K.sub.s, and the spring constant K.sub.s
of the springs being substantially greater than the spring constant
K.sub.t of the tubes, thereby spacing the tubes apart and
substantially precluding vibration of the tubes.
4. The improvement of claim 3, wherein
5. The improvement of claim 3, wherein the first and second spring
each has a maximum elastic deflection which exceeds the deformation
necessary during installation by insertion between the first and
second tubes of the support bar carrying the first and second
springs, thereby assuring a resilient engagement of the first and
second tubes, respectively, and ease of assembly.
6. The improvement of claim 1, wherein the plurality of hairpin
springs carried by said support bar is fabricated from one piece of
sheet metal by stamping a long narrow stem and further stamping
pairs of legs extending substantially perpendicular to the stem, by
folding the stem to form the hairpin bight where pairs of legs join
and an integral stiff channeled spacer between pairs of legs, by
bending each leg to form a bow-shaped spring which is bowed outward
from the support bar, and by securing the other ends of the legs to
the support bar by, for example, spot welding.
7. In a heat exchanger having a plurality of spaced-apart tubes and
further having at least one support bar disposed intermediately of
the respective tubes, the improvement in reducing vibration of the
tubes against the support bar, comprising a plurality of pairs of
bifurcated legs having respective end portions and being joined
together at a common bight portion, the bight portion being secured
to the bar, and wherein each end portion resiliently engages a
first tube and a second tube respectively, thereby substantially
precluding vibration of the tubes.
8. The improvement of claim 7, wherein the maximum elastic
deflection of the spring legs exceeds the deformation during
installation whereby assuring a resilient engagement of the first
and second spring with the first and second tubes respectively, and
ease of assembly.
9. An apparatus for supporting a flow tube in the U-bend of a
bundle of tubes in a heat exchanger, wherein the flow tube has a
spring constant K.sub.t and further has a pair of substantially
diametrically-opposed sides, and wherein a pair of support bars
including a first bar and a second bar is provided, such that the
tube is disposed between the first and second bars, comprising a
first spring carried by the first bar and resiliently engaging one
side of the tube, a second spring carried by the second bar and
resiliently engaging the other side of the tube, the springs having
a substantially equal spring constant defined as K.sub.s, and the
spring constant K.sub.s of the springs being substantially greater
than the spring constant of K.sub.t of the tube, such that the tube
is supported and substantially prevented from vibrating, and
further comprising a plurality of hairpin springs, each hairpin
spring having two legs, each leg having a bight, a middle and end
portions, wherein the first and second springs carried by the first
and second support bars are each a bowed middle portion of a
hairpin spring leg, the end portion being secured to the support
bar and the bight portion straddles the edge of the support
bar.
10. The apparatus of claim 9, wherein
11. An apparatus for supporting a flow tube in the U-bend of a
bundle of tubes in a heat exchanger, wherein the flow tube has a
spring constant K.sub.t and further has a pair of substantially
diametrically-opposed sides, and wherein a pair of support bars
including a first bar and a second bar is provided, such that the
tube is disposed between the first and second bars, comprising a
first spring carried by the first bar and resiliently engaging one
side of the tube, a second spring carried by the second bar and
resiliently engaging the other side of the tube, the springs having
a substantially equal spring constant defined as K.sub.s, and the
spring constant K.sub.s of the springs being substantially greater
than the spring constant of K.sub.t of the tube, such that the tube
is supported and substantially prevented from vibrating, and
wherein the first and second springs carried by the first and
second support bars respectively are each one leg of a pair of
bifurcated legs joined together at a common bight, and the bight
portion is secured to the respective bar.
12. The method of assembling a substantially vibration-free support
for flow tubes in the U-bend of a heat exchanger comprising the
steps of providing a plurality of spaced-apart flow tubes,
providing a plurality of support bars, providing springs disposed
on and secured to the support bars, providing springs disposed on
and secured to the support bars, each spring having a pair of
bifurcated legs exerting a resilient force on its respective flow
tube which eliminates any clearance gap and substantially precludes
vibration, inserting the support bars carrying the springs between
columns of tubes, flexing each spring as it encounters each tube
before reaching the installed position, and securing the support
bar in position once each spring is resiliently engaging the
correct flow tube.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of steam generators for nuclear
power plants and in particular to apparatus for providing resilient
support of flow tubes in the U-bend of the tube bundle in a nuclear
power steam generator, thereby eliminating gaps between the tubes
and the supports and substantially reducing vibration of the tubes
due to the motion of fluids.
2. Description of the Prior Art
In a commercial nuclear power plant, thermal energy from the
nuclear reactor is carried by the reactor coolant to a heat
exchanger in which the delivered heat converts the feedwater into
steam. The steam drives the turbine which converts the
thermodynamic power into mechanical power. A mechanical coupling
between the turbine and a generator delivers mechanical power to a
generator which converts mechanical power into electric power.
In the heat exchanger, the reactor coolant flows through a bundle
of flow tubes. The bundle has a straight portion followed by a 180
degree "U-turn" after which there is a second straight portion to
the manifold which gathers the reactor coolant for return to the
reactor for reheating.
In order to maximize the heat flow from the reactor coolant to the
working fluid of the steam generator, a large number of small
diameter thin wall tubes are contained in the bundle, and the
spacing between the tubes assures good heat transfer. Each flow
tube is readily set into vibration by the motions of steam and
water. Such vibration, if unrestrained, will lead to the
destruction of the tube. Consequently, anti-vibration bars have
been introduced in the prior art.
The vibration problem is particularly acute in the U-bend of the
flow tubes. The anti-vibration bar of the prior art comprises a bar
bent into a V-shaped configuration such that two legs are formed
with an angle between. The V-shaped anti-vibration bars are
inserted between successive columns of the steam generator flow
tubes. The ends of the V-shaped anti-vibration bars are connected
to the respective ends of bars inserted into the adjacent gaps
between columns of tubes. These anti-vibration bars limit the
excursion of vibrations, but since there is at least a clearance
gap between the bar and every flow tube, a vibration and rattle
occurs as the flow tube is driven by the motion of steam and water
around it. The result is a fretting and corrosion of flow tubes
which leads to subsequent failure and expensive repair.
U.S. Pat. No. 4,285,396 issued to F. Schwoerer on Aug. 25, 1981 and
assigned to Wachter Associates, Inc. discloses spring collar
devices positioned at intervals along the tubes which permit
lateral movement of the tubes but prevent any relative movement
between the tube and its associated spring collar device. Any
lateral movement is between the adjacent spring collar devices.
With this arrangement fretting and corrosion between the outer
surfaces of the tubes and their lateral support members are
substantially eliminated. This system is for new nuclear steam
generators.
U.S. Pat. No. 4,337,827 issued to F. S. Jabsen on July 6, 1982 and
assigned to Babcott & Wilcox Company, discloses a support for a
helically coiled fluid heat exchanger. In this heat exchanger flow
tubes are bent in helical shapes and adjacent turns of the helix
are nested in a plurality of support members. Pairs of tubes are
nested between support members against a support strip, a spring is
placed over the tubes and a second strip and is pressed on the
assembly to the desired spring pressure and affixed. The method of
vibration elimination of invention '827 is not applicable to
standard nuclear steam generator flow tube geometries.
U.S. Pat. No. 4,747,372 issued to R. M. Wepfer et al on May 31,
1988 and assigned to Westinghouse Electric Corp. discloses an
apparatus for minimizing the gaps between anti-vibration bars and
the flow tubes in the U-bend section of a nuclear steam generator.
Anti-vibration bars are provided between columns of flow tubes in
the U-bend region and these bars are contoured on one side to
correspond to the exact as built and as bent diameter of the tubes
as located within the tube bundle. Such custom fitting of tube
anti-vibration bars to actual tube size reduces the gap between
anti-vibration bar and tube, but the fitting does not completely
eliminate such gaps.
There still exists a need for a better apparatus and method for
eliminating the gaps between flow tubes and support bars, for
minimizing the vibration of flow tubes and for substantially
eliminating the fretting and corrosion of flow tubes due to the
action of fluids in a heat exchanger.
Accordingly, one object of the present invention is to provide an
apparatus and a method which eliminates gaps between tubes and
support bars and thereby precludes a rattling vibration. Another
object is to resiliently but firmly support the flow tubes in the
correct position in the nuclear steam generator. A further object
of the invention is to provide a means of gap elimination and
vibration reduction which is convenient to install in both existing
and new nuclear power steam generators and is a means which is cost
effective.
SUMMARY OF THE INVENTION
The present invention substantially eliminates vibration, fretting
and corrosion of flow tubes in the U-bend section of the tube
bundle in a steam generator. In accordance with the teachings of
the present invention, there is herein and described an apparatus
for the support and spacing of flow tubes in the U-bend of a steam
generator having a plurality of flow tubes disposed in a regular
pattern of columns of tubes with a space between the columns. Each
tube has a first straight portion, a U-bend and a second straight
portion. The semi-circular segments of flow tube in the U-bend are
flexible and without support are unable to maintain position with
an acceptably small amplitude of vibration. A plurality of support
bars are inserted between the flow tubes columns to provide
substantially rigid support and control spacing. Each bar, having a
clearance gap with respect to the column of tubes on each side, has
a plurality of hairpin springs secured to the bar and disposed at
positions to resiliently engage the flow tubes in the columns on
both sides of the bar. The springs are characterized by spring
constant K.sub.s and the tubes flexure elasticity is characterized
by a spring constant K.sub.t. A value of the spring constant
K.sub.s which is five times the tube elastic flexure constant
K.sub.t reduces vibration to an acceptable value, eliminates
fretting and corrosion due to gaps between tube and support and
facilitates convenient installation by elastic accommodation as
needed during insertion of the bar.
A substantial number of existing nuclear power plants have steam
generators which contain tube bundles geometrically organized in
orderly columns. Prior art solutions to the problems of vibration,
corrosion and fretting have not been completely satisfactory. The
embodiment of the present invention in an existing steam generator
will substantially reduce the vibration, corrosion and fretting of
flow tubes in the U-bend section thereby reducing the maintenance
cost in the production of electric power.
The present invention further teaches a method of manufacture of
the plurality of support bars with the respective plurality of
springs which resiliently engage the flow tubes. The substantially
rigid strength member is a rectangular cross-section bar which fits
with clearance between columns of tubes. The hairpin springs for
disposition along the bar at the proper locations to engage flow
tubes are manufactured by stamping both the hairpin springs and the
spacing stem as one integral piece. The hairpin springs are formed
by folding the legs of the spring together forming a bight in both
the spring and the stem between springs. The spring assembly is
then clipped onto the support bars so that the spring legs straddle
the support bar and the bight wraps around one edge of the support
bar. A bow is bent in each leg of each spring to provide the
necessary spring for resiliently engaging the flow tubes. The end
of the leg is secured to the bar by spot welding. The method of
manufacture is a simple and cost effective process for providing a
means for the reduction of vibration, fretting and corrosion of
flow tubes in the U-bend section of existing and new steam
generators.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view, partially in cross section, of a
nuclear steam generator having a U-bend section of the flow tube
bundle for the embodiment of the present invention which
substantially reduces vibration, corrosion and fretting.
FIG. 2 is a schematic drawing of the U-bend section of the tube
bundle showing only three ranks of semi-circular flow tube in order
to clearly illustrate the prior art anti-vibration bars. The
V-shaped anti-vibration bars are inserted between the columns of
tubes, the spread of the V-shape becoming smaller for the higher
vibration bars.
FIG. 3 schematically illustrates an embodiment of the invention in
which support bars, each carrying a plurality of hairpin springs,
are inserted between columns of flow tubes. Hairpin springs with
bowed legs resiliently engage the flow tubes when the support bar
is in the correct position.
FIG. 4 shows one side of the support bar and sections of several
flow tubes near the bar.
FIG. 5 schematically illustrates the hairpin spring with two legs
bowed to resiliently engage flow tubes on either side of the
support bar. FIG. 5 is section 5--5 of FIG. 4.
FIG. 6 is a schematic cross section of the support bar between
hairpin springs showing how the bight in the stem clips onto one
edge of the bar. FIG. 6 is section 6--6 of FIG. 4.
FIG. 7 schematically illustrates the manufacture of the springs and
spacer stem between springs. Hairpin springs and stems between
springs are stamped as one integral piece from sheet metal. The
piece is folded 90 degrees along the two dashed lines and clipped
on the support bar so that the legs straddle the bar as illustrated
in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the operation of a nuclear power plant
involves heating a coolant in a nuclear reactor (not shown) and
transporting the hot coolant to a heat exchanger wherein water is
heated to form steam. The steam generator 10 is enclosed in a
pressure vessel, the wall 11 of which is shown in cross section.
Hot coolant from the reactor enters inlet 12 and is distributed
among the tubes of the bundle 14 rises up through the straight
section, makes the U-turn 15 and then returns in a second straight
segment to the coolant return manifold separated from the inlet
manifold by partition 18. The coolant is then delivered back to the
reactor for reheating via outlet port 13. Feed water for the
production of steam is introduced at inlet 16. Water falling on the
flow tube bundle is turned into steam, and, after a separation of
water from the steam, the steam is delivered to the turbine through
outlet 17.
Referring to FIG. 2, the plurality of semi-circular segments of
flow tube in the U-bend section of the tube bundle is shown. In
FIG. 2, three ranks of flow tube 21, 22 and 23 are shown to
illustrate the geometry of prior art solution to the vibration
problem, namely the V-shaped anti-vibration bars 24, 25 and 26. In
practice the density of tubes would preclude illustration of the
prior art anti-vibration bar geometries, such close spacing of
tubes is shown by the column of tubes labelled 29. While the prior
art anti-vibration bars shown in FIG. 2 provide a restraint against
vibration, the necessary clearance between the bar and the tube for
installation leads to a rattling of the flow tube against the bar
with a resulting fretting, corrosion and even mechanical fatigue
due to persistent pounding. In the present invention a support bar
is inserted between the columns of tubes, and according to the
teaching of the present invention, the support bar exerts a
resilient positioning and spacing force against each tube via a
respective spring attached to the support bar.
Referring to FIG. 3, the embodiment of the invention 30 is a
plurality of support bars 31 inserted between columns of flow
tubes, column 29 is sectioned to show the bars. Mounted on each
support bar is a series of bow-shaped springs 36, 43 which
resiliently engage the flow tube on either side of the support bar
and which are secured to the support bar by spot welding 37. The
material and the design of the bow spring is selected such that the
elastic constant of the bow spring K.sub.s is equal to five times
the bending elastic constant for the tube K.sub.t. With this
relationship between the elastic constants, the flow tubes 29 are
securely spaced and disposed, and at no time is there a gap between
the flow tube and the support spring.
Referring to FIG. 4, the support bar 31, flow tubes 32-35 and the
bow spring 36 are shown in more detail. The bight 45 forming the
hairpin spring clips onto the edge of the support bar 31. The end
of the bow spring leg of the hairpin is spot welded 37 to the bar
31.
Referring to FIG. 5, a cross section through 5--5 of FIG. 4 is
shown. Flow tubes 41 and 42 are resiliently engaged by bow springs
43 and 44, respectively. Each spring is formed by shaping a bow in
a leg of a hairpin spring which straddles the support bar 31. The
legs of the hairpin spring are spot welded 37 to the support bar
31. The hairpin springs are formed by bending an appropriately
stamped piece of sheet spring stock to form a bight 45 around one
edge of the support bar 31.
Referring to FIG. 6, which is section 6--6 of FIG. 4, the stem
portion 46, 59 which carries and spaces the hairpin springs is
clipped to one edge of the support bar between hairpin springs.
Referring to FIG. 7, the hairpin springs 51-56 and the spacer stem
59 between them are stamped from sheet spring stock to form an
integral unit 50. The lateral portions of the stamping 51-56 form
the respective pairs of legs of hairpin springs. The unit is
fabricated by folding a 90 degree bend along the two dashed lines
57 and 58. The spring legs 51-56 are further bent to form a bow
prior to securing the ends of the legs by spot welding 37 as shown
in FIGS. 3, 4 and 5. The stem 59 disposes the springs at the
appropriate position to engage the flow tube on either side of the
support bar.
* * * * *